Screws come in a variety of different configurations generally including threads on a portion of a substantially cylindrical shaft which extends between a head and a tip. One form of screw is configured for use along with a nut or other structure having a threaded hole, which nut threads mate with threads on the screw so that the nut or other threaded structure can be coupled to the screw by simultaneously rotating and translating the shaft into and through the hole in the nut or other threaded hole. When a screw is configured for use with a nut it is often referred to as a bolt.
The threads on the shaft of the screw have a variety of characteristics. Some of these characteristics include a helix angle, also called a pitch angle, which is an angle at which the threads are oriented relative to a plane perpendicular to a central axis of the shaft of the screw. The greater the pitch angle, the greater the axial distance that a nut will travel when rotated a similar number of turns. Another characteristic of threads is their size, such as measured between a minor diameter and a major diameter of the threads. Threads which are generally larger are often referred to as “coarse threads” while threads which are relatively smaller are often referred to as “fine threads.” When adjacent turns of threads are directly adjacent to each other, and when a diameter of the shaft of the screw is known, the pitch angle of the threads and the relative size of the threads become linked together. Coarser threads will have a greater pitch angle and finer threads will have a lesser pitch angle if the diameter of the shaft remains the same and if the threads of adjacent turns are provided without a space therebetween.
One unique type of screw is known in the prior art as a dual threaded screw. Generally, a dual threaded screw has both a coarse thread with a first relatively large helix angle and at least portions of a second fine thread having a lesser helix angle formed on the same screw. Such a dual threaded screw is described in U.S. Pat. No. 7,159,429, incorporated herein by reference in its entirety. A dual threaded bolt B providing one example of such a dual threaded screw is shown herein in FIGS. 1-6. A unique aspect of such a dual threaded bolt B is its ability to have nuts N, L having threads of different pitches both threaded onto the same bolt B with the first nut N providing a basic fastening function, and the second nut L being tightenable against the first nut N to act as a lock nut to keep the nuts N, L from moving relative to the bolt B.
Screws can be manufactured in a variety of different ways including machining of the threads, turning of the threads on a lathe or rolling of the threads between dies. Often, rolling threads onto a screw is the most efficient screw thread forming method. However, dual threaded screws do not lend themselves to straightforward roll forming between appropriately threaded dies. In particular, if one sequentially utilizes coarse threaded dies and then fine threaded dies, one can form the coarse threads onto the screw, but then when the fine threads are formed onto the screw with the fine threaded dies, the coarse threads are to some extent damaged. If the fine threads are first formed onto the shaft of the screw and then the coarse threads are formed, the dies forming the coarse threads will damage the fine threads.
It is taught in the above-identified U.S. Pat. No. 7,159,429 to provide a die having contours thereon which are appropriate to result in the formation of both the coarse thread and the fine thread, simultaneously onto the shaft of the screw. The particular geometry of such a roll forming die is described in U.S. Pat. No. 7,159,429.
While this die geometry for forming the dual threaded screw is disclosed in U.S. Pat. No. 7,159,429, this die geometry is rather complex and is of a character which makes forming of this die exceptionally difficult. This difficulty is enhanced when considering that materials from which roll forming dies are formed must be significantly harder than the material roll formed thereby. Because screws are typically formed of steel or other relatively hard materials, the dies must be formed of exceptionally hard material (e.g. tool steel) and yet must still have the complex geometry necessary to simultaneously form both coarse and fine threads. Harder materials are harder to form into their required shape, for instance requiring grinding instead of cutting.
Accordingly, a need exists for a method to form the complex geometry of a die suitable to roll form a dual threaded screw.